In some applications involving the use of hydrogen, or gas mixtures containing hydrogen, contamination of the process gas (or gases) can occur due to the presence or generation of organic gases, organic vapors, organic mists, or particulate matter during the particular processing application. And, if the initial, relatively pure, process gas (or gas mixture) is used in large volumes, purification and re-use of this gas may be an economic necessity.
Although there is an extensive body of literature covering varying methods of purifying gases, many of these methods are often problematic in dealing with relatively high concentrations of organic contaminants in gas streams containing high concentrations of hydrogen. For example, membrane purifiers can easily and rapidly become so contaminated themselves by the removal of organic vapors and oil mists, that they quickly become ineffective. Even the use of pre-filtration (for example, standard types of cartridge filters or activated carbon beds) to protect membrane type purifiers is often not effective for very long when there are high levels of organic mists or high molecular weight oil contamination within the gas(es) so purified. These kinds of pre-filtration/adsorption schemes can sometimes lead to frequent maintenance or complete replacement of the active filtering means and can also sometimes lead to irreparable deterioration in membrane elements if the contamination eventually “breaks through” any of the pre-filtering devices. One proposed solution includes that disclosed by Kidnay, A. J., Hiza, M. J., and Dickson, P. F., “The Kinetics of Adsorption of Methane and nitrogen from hydrogen Gas”, and “Advances in Cryogenic Engineering”, Vol. 14, K. D. Timmerhaus (Editor), plenum Press, NY 1969, pp. 41-48 (hereinafter, Kidnay et al.).
Another frequently used method of purifying gases, such as hydrogen or helium, involves cryogenic trapping of impurities entrained within these gases. In this kind of process, contaminants are removed by condensation, or adsorption, or by “freezing out” as solids within a low temperature adsorption bed. Often, at least one adsorption bed employed in using this kind of technique involves the use of activated carbon (or activated charcoal, zeolitic molecular sieves, activated alumina, silica gels, and the like, as well as combinations of these conventional adsorbents) in a low temperature adsorption process [Kidnay et al.]. The main problem with this approach is that it is difficult to regenerate conventional packed bed adsorbents that become saturated or nearly saturated with high molecular weight organic impurities. Typically, high temperature steam must be used in these cases, and then an involved process of moisture removal by inert gas purging, at high temperatures, must follow that kind of regeneration step.
Many adsorbents are used in the field of gas separation, one of which includes silica gel. Silica gel is a granular, highly porous form of silica (SiO2). Generally speaking, it is formed by reaction of a sodium silicate solution with a mineral acid such as HCI or H2SO4, followed by polymerization of the produced hydrosol. Because of the —OH functional groups, silica gel is a relatively polar material. On the other hand, porous glass is a relatively less polar material in comparison to silica gel.